This invention relates to RF admittance measuring systems for monitoring the condition of materials, and more particularly, to systems of this type which are adapted for use at remote locations.
Heretofore, two-wire transmitters have been utilized to monitor various conditions at a remote location. Typically, a two-wire transmitter at a remote location is connected in series with a power supply and a load at another location through two transmission wires. As the condition being monitored at the transmitter varies, the effective series resistance across the transmitter varies so as to produce a change in the current drawn by the transmitter which represents (e.g., is generally proportional to) the condition being monitored. A two-wire transmitter of this type is designed for low power consumption since the amount of power available to the transmitte from the remotely located power supply may be limited. Furthermore, certain applications may require that the two-wire transmitter be "intrinsically safe" so as to permit its use in the monitoring of conditions in an explosive environment. Under these circumstance, lower energy usually associated with lower power consumption becomes important so as to preclude the possibility of ignition and explosion.
Although the state of the art in two-wire transmitters is adequate for monitoring various types of conditions, the prior art technology with respect to the RF admittance measurement is deficient for two-wire transmitters for the following reasons.
When measuring the RF admittance between a probe electrode and a reference surface such as a grounded vessel, the resistance is parallel with the capacitance between the probe electrode and the grounded vessel becomes very important from a power consumption standpoint. Heretofore, it has generally been assumed that shunt resistance is sufficiently small in a sufficiently large number of applications so as to render the power provided by the 4 milliamp current in a 4-20 milliamp two-wire transmitter system insufficient to power the two-wire transmitter. In other words, the shunt resistance alone might consume more power than is available at the 4 milliamp condition leaving little or no power to operate the circuitry of the transmitter. Also, power limitations exist where the admittance measuring circuit is battery-powered.
Moreover, in order for an admittance measurement to be accurate, reliable detection must be utilized. However, such reliability usually requires a substantial source of power which is inconsistent with the low power requirement of a two-wire transmitter as discussed above and the available power because of the shunt resistance. This combination of factors imposes severe restrictions on the power which is generally considered necessary to provide a reliable RF signal source. Similar restrictions are placed on the power generally considered necessary to assure that the detector operates with a high degree of reliability.
Another problem which is somewhat unqiue to admitance measurements is the isolation of the admittance responsive network in which the unknown admittance being measured is connected. Typically, the unknown admittance being measured is from a probe electrode to ground as disclosed in Maltby et al U.S. Pat. No. 3,781,672 and Maltby U.S. Pat. No. 3,706,980, both of which are assigned to the assignee of this invention. However, a power supply at a location remote from the admittance responsive network as in the case of a two-wire transmitter, may not be connected to ground in a manner compatible with the network. It is therefore necessary to isolate the admittance responsive network, or at least the admittance sensing probe, from the power supply so as to permit the network to be connected to ground regardless of the power supply circuit. This is also true of the admittance responsive networks employing a variable frequency oscillator such as that disclosed in Spaw U.S. Pat. No. 3,807,231. Moreover, if the voltage across the unknown admittance were reduced to minimize power consumption, the signal representing the changes in admittance of the admittance responsive network would require amplification. Accordingly, the problem exists of providing an isolated source of power for such amplification.
Other problems exist in assuring linear and stable calibration of the admittance measuring system. It is also important to provide a system which will work with various types of probes and various lengths of cables associated with the probes without adversely affecting the admittance measurement.
Another problem which can be quite troublesome is the low level of analog signals which may be generated by an admittance measuring system. Low level analog signals are particularly difficult to process if a high degree of accuracy is to be attained.
To a very large degree, the above-mentioned problems are encountered when the system for monitoring the condition of materials comprises a battery-operated unit as well as a two-wire transmitter. Under these circumstances, the available power is again limited.